Hi Adrian Thanks for the good suggestion with example.I tried the same with regular expression.
showing sample
(match_regex($POSTCODE, '[A-Z][0-9]{2} [0-9]{1}[A-Z]{2}', NULL)
match_regex($POSTCODE, '[A-Z]{2}[0-9] [0-9]{1}[A-Z]{2}', NULL)
match_regex($POSTCODE, '[A-Z][0-9][A-Z] [0-9]{1}[A-Z]{2}', NULL))
And Thanks for the link also.